An <u>onion </u>seller is waiting for customers at the market.
Explanation:
So according to the expectation of the question, the answer "onion", "o" is not capitalized.
Reason:
"Onion" is a noun. In simple terms noun represent "name, place, animal things".
So noun should be capitalized but it also depends on the type of noun. All the vegetable names, fruits name, etc comes under common noun.
Example: cat, women, uncle
So, mostly common nouns are not capitalized.
But Proper nouns are usually capitalized.
Example: Pacific Ocean, America
